Prior Art (1)
A video file data distribution service using the Internet is being provided. FIG. 1 shows a diagram depicting this system, where data is directly distributed from a plurality of distribution servers 100 to client PCs (personal computers) 101 via the Internet 102.
The general protocols for distributing data in the system in FIG. 1 are as follows.    (1) http protocol (protocol used for a www server)    (2) ftp protocol (protocol used for a file server)    (3) rtsp protocol (real-time streaming protocol, protocol used for streaming formats)
The size of a video file is very large. If the file is a giga byte size, the load of the distribution server and the band consumption of the network lines are so large that it requires an enormous equipment investment and running cost if the data is distributed to many users simultaneously using a prior art.
In the above prior art (protocol), a server and a client PC (personal computer: viewer, user) are directly connected, and occupy the resources of the server and the line, so a cost in proportion to the number of users is required.
Prior Art (2)
One method to solve the above problem is a kind of P2P protocol (Peer-To-Peer, method for clients to communicate with each other without passing through a server) called the “Bit Torrent Protocol”. As FIG. 2 shows, in this method files are transferred relaying packets from the server having the original file called a “super seeder” (also called the origin server) 110 as a start. Each server, which is called the leecher 111, distributes a portion, received as a piece seeder from the received portion, to other peers. A client which received at least a part of the file plays a role to distribute this portion, and is called the piece seeder. A peer which only receives is called a leecher hill 112. The leecher hill 112 responds to the client PC 101 in FIG. 1, and the piece seeder (leecher) 111 is the client PC 101 in FIG. 1, but also corresponds to the distribution server 100. The piece seeder (leecher) 111 transmits for the number of times of the reception, then the piece reaches to the last client. (Hereafter similar methods are collectively called the “Bit Torrent Method”. “Packet relay distribution” is hereafter called the “grid distribution”.)
The URL related to the Bit Torrent Method has the extension “.torrent”. If this .torrent file is read by Bit Torrent-supported file sharing software, file sharing (download/upload of a file) starts. Even during the download of a file, the portion this client has downloaded thus far is uploaded when requested.
In theory a file reaches all the clients 111 and 112 if the seeder server 110 distributes the file once, regardless the number of clients, so the load on the seeder server 110 for one file is roughly constant regardless the number of clients. (In reality the distribution efficiency of an entire network is improved by executing not just once, but a plurality of times of distribution to a plurality of peers.)
FIG. 3 shows a diagram depicting a Bit Torrent system. To actually implement the Bit Torrent Method, a central control server called a tracker 120 exists to register and manage addresses of the peers (client PCs) P which participate in the grid distribution. The tracker server 120 receives a network participation request from each peer P, and creates a list of addresses (IP address and port) of the peers P which sent that request for each identifier (file ID) which uniquely specifies the file desired by a peer. This list is called the active peer table 120a (FIG. 4).
To participate in the network 121 in FIG. 3, the peer P first inquires the tracker server 120 to know the connection destination client of the grid distribution, and acquires the address of the connection destination peer P. At this time the address of the inquiry source peer P (IP address and port) and the identifier (file ID) which uniquely specifies the file desired by this peer P are registered in the active peer table 120a. The peer P which acquired the address of the connection destination by the response from the trackers server 120 connects with the corresponding peer P, and hereafter a packet relay type communication (grid distribution) is performed between the peers, as shown in FIG. 2. In FIG. 3, the bold line indicates the data communication and the fine line indicates the communication with the tracker server 120 for registration to the active peer table 120a. 
The peer P is a client PC but can also be a seeder (seeder or piece seeder) as well. In other words, when a specific file is downloaded 100% (when file A or file B is downloaded 100% if one content file consists of file A and B), the client PC becomes a seeder, and is managed by the active peer table 120a as such. In the network 121 in FIG. 3, each node is indicated as “peer P” since whether the node is a seeder or not depends on the node. Each peer P of the network 121 is a seeder or a simple client (leecher) for a specific file. Status differs depending on the file and is not fixed, and a peer P that is a seeder for a specific file can be known by referring to the active peer table 120a. 